1. Field of the Invention
The present invention relates to a Gigabit Ethernet passive optical network (GE-PON) having an optical line terminal (OLT) and a plurality of optical network units (ONUs) and, more particularly, to a GE-PON having a double link structure to address a link failure between the OLT and each ONU, and a double link method using the same.
2. Description of the Related Art
A passive optical network (PON) is a communication network that transfers signals to end users over an optical cable network. The PON consists of a single OLT installed in a communication facility and a plurality of ONUs installed nearby the subscribers, typically a maximum of 32 ONUs connectable to one OLT.
In the PON, the OLT transmits a signal to an ONU via an optical fiber. The ONU receives the signal transmitted from the OLT, processes it in a predetermined manner and then transfers the processed result to an end user. Thus, the ONU is an optical network termination unit that provides a service interface to the end user.
The ONU accommodates FTTC (Fiber To The Curb), FTTB (Fiber To The Building), FTTF (Fiber To The Floor), FTTH (Fiber To The Home), FTTO (Fiber To The Office), etc. In this type of connection, the ONU is implemented to provide a high service accessibility to the subscriber. The ONU functions to interconnect a cable connecting the subscriber for the transmission of an analog signal therefrom, and an optical equipment connecting the OLT for the transmission and reception of optical signals thereto/therefrom. Accordingly, the ONU performs an optical/electrical conversion operation to convert an optical signal from the OLT into an electrical signal for subsequent transmission to the subscriber, as well as an electrical/optical conversion operation to convert an electrical signal from the subscriber into an optical signal for subsequent transmission to the OLT.
FIG. 1 shows a downstream data transmission structure of a Gigabit Ethernet passive optical network, and FIG. 2 shows an upstream data transmission structure of the Gigabit Ethernet passive optical network.
As shown in FIGS. 1 and 2, the Gigabit Ethernet passive optical network (GE-PON) has a structure of one OLT 10 connected to a plurality of ONUs 20, 22 and 24 in a tree form via an optical splitter 15. The GE-PON is considered to be less expensive and more efficient than an AON (Activity-On-Node) network.
Prior to the development of the GE-PON, an asynchronous transfer mode passive optical network (ATM-PON) has been developed and standardized. The ATM-PON transmits ATM cells in the form of a block in the upstream or downstream direction. Also, an Ethernet passive optical network (E-PON) has been developed to transmit packets of different sizes in the form of a block. As such, the E-PON has a complex control structure compared with the ATM-PON.
Referring to FIG. 1, during the downstream transmission, the OLT 10 broadcasts data to be transmitted to the ONUs 20, 22 and 24. The optical splitter 15 receives the data broadcast from the OLT 10 and transmits the received data to each of the ONUs 20, 22 and 24. Each of the ONUs 20, 22 and 24 detects data to be transferred to a corresponding one of users 30, 32 and 34 via the optical splitter 15 and receives only the detected data to the corresponding user 30, 32 or 34.
Referring to FIG. 2, in the upstream transmission, data from the users 30, 32 and 34 are transferred to the ONUs 20, 22 and 24, respectively. The ONUs 20, 22 and 24 transmit the data from the users 30, 32 and 34 to the optical splitter 15 according to a transmission permission protocol from the OLT 10, respectively. At this time, each of the ONUs 20, 22 and 24 transmit the received data during a time slot set in a TDM (Time Division Multiplexing) manner. Therefore, there is no data collision resulting during the upstream data transmission in the optical splitter 15.
However, the OLT 10 and each of the ONUs 20, 22 and 24 are interconnected by a single link set up therebetween via the optical splitter 15. As a result, if a failure occurs in the link set up between the OLT 10 and each of the ONUs 20, 22 and 24, it is impossible to perform the communication and requires a large amount of time to recover from the link failure.